util/abuild: Enable abuild to compile a single variant

util/abuild: Enable abuild to compile a single variant

There are many boards in coreboot which support multiple
variants. When abuild is used to compile a single target, it builds
all its variants. If a target has 5 variants, then abuild takes nearly
10x the time to compile all variants of the target. This change adds
an option -b/--board-variant to enable abuild to compile only a single
variant of the target.

TEST=Verified:
1. abuild builds all variants of the target if -b option is not
provided.
2. abuild builds a single variant if -b option is provided.
3. abuild prints appropriate error message if invalid variant name is
provided.

Change-Id: I3781568c6409c5ec2610a8386a21d86037428e7f
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/27215
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Justin TerAvest <teravest@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>